Cracking of olefins



Patented Oct. 21, 1947 CRACKING F OLEFINS Francis 0. Rice, Washington, D. C.

No Drawing. Application April 1, 1942 Serial No. 437,275

4 Claims.

This invention relates to processes for cracking liquid or normally gaseous olefins; and more particularly has reference to an improvement in such processes to obtain increased yields of diolefins and acetylenes and to methods of separating the products obtained.

In the prior art, the cracking of olefins is accompanied by the production of tars, heavy oils and carbon together with other products of little commercial value and the yields of diolefins and acetylenes have been relatively small. For example, Tropsch, Parrish, and Egloff, J. Ind. Eng. Chem., 28,581 (1936) made a study of the hightemperature pyrolysis of normally gaseous olefins and reported that liquid and carbon were observed in all cases. Furthermore, in the pyrolysis of isobutylene, they reported the presence of acetylene and butadiene among the products but did not find any methyl acetylene or allene.

An object of this invention is to provide a method of cracking olefins to obtain increased yields of diolefins and acetylenes.

Another object of this invention is to provide a method of cracking olefins to obtain increased yields of diolefins and acetylenes in which the cracking reaction is carried out at a high temperature under a sub-atmospheric pressure and at a rate such that no appreciable amount of tar, oil, smoke or carbon is formed.

Still another object of this invention is to provide a method of cracking olefins to obtain increased yields of diolefins and acetylenes in which the cracking reaction is carried out under conditions of pressure, temperature and time approaching, but short of those at which appreciable amounts of tar, liquid, smoke and carbon are contained in the reaction products.

A further object of this invention is to provide a method of cracking olefins to obtain increased yields of diolefins and acetylenes and to effect separation of the reaction products.

With these and other objects in view, this invention comprises the steps hereinafter set forth.

In accordance with the present invention, monolefins, either liquid or normally gaseous or both, are heated under subatmospheric pressures for such a time that no appreciable formation of carbon, tarry or oily substances is effected. Under these conditions which may be determined rather easily by experimentation, it has been found that the yields of diolefins and acetylenes are much higher than formerly obtained by cracking olefins.

In order to determine the conditions of pressure and rate of flow for optimum yields of the reaction products, the present invention contemplates an actual visual inspection of the reaction products to determine whether or not they contain any appreciable amount of carbon, or tarry, or oily substances. For instance, starting with a given temperature, pressure and rate of flow of the olefins through a cracking zone, I have found that by gradually increasing the pressure and the rate of flow or both, such conditions will be reached that upon inspecting the reaction products, it will be found that visible tar, carbon, oil or smoke formation has begun. After these conditions have been reached, the pressure is gradually dropped, or the rate of flow gradually increased, or both, to a point just below the conditions at which tar, carbon, oil, or smoke formation began. When these modified conditionsare reached, they are noted as they will be the optimum conditions for obtaining a high yield of the desired reaction products.

The conditions for no smoke, tar, oil, or carbon formation depends in a somewhat undetermined manner upon the pressure, temperature, rate of flow of the vapor, size and nature of the cracking zone, and the optimum conditions can best be determined by experiments in the manner pointed out above rather than by absolute limits of pressure, temperature, rate of flow, size and condition of cracking zone, etc. It is possible, however, after experience with crackin zones of a given type to determine by analogy, the optimum conditions without the need of repeated preliminary experiments on each new cracking zone employed.

As an example, I have carried out an experiment in which 50 grams of isobutylene were passed through a clean quartz tube, about 1 inch in diameter and 10 inches long, heated to 773 C. At the beginning, the rate of flow of the isobutylene' vapor was rather low and the pressure within the tube was of the order of 10 mm. The pressure and the rate of flow of the isobutylene through the tube were then slowly and progressively increased, while observing the dischargeend of the tube through a window provided for the purpose, until the pressure and the rate of flow were raised to such a point that tar, carbon, oil or smoke formation had begun and was visible through the window. After reaching this point in the procedure, the pressure was slowly reduced and the rate of flow was slowly increased, or both, to a point just below the conditions in which tar, carbon, oil or smoke formation began to be visible through the window at the discharge end of the tube.

These conditions will vary as hereinbefore pointed out, depending on the size of the tube and the nature of the tube walls, and for each new tube employed, especially of a type not previously used, an experiment such as above pointed out must be carried out to determine the optimum operating conditions.

In carrying out the cracking reaction in the quartz tube about 1 inch in diameter and inches long at 773 C., it was ascertained that with a pressure of about /7 of an atmosphere and a rate of flow that the contact time of the olefin with the heated quartz tube was about 0.46 second, no formation of tar, oil, carbon, or smoke was visible, the tube remaining clean after completion of the experiment.

Under the optimum conditions described above,

the cracking of isobutylene was efiected and it was found that for each mol of isobutylene 'decomposed, there was obtained 0.21 mol of allene together with 0.38 mol of methyl acetylene. The above mentioned quantities of allene and methyl acetylene together with various amounts of propylene, methane and hydrogen, and undecomposed butylene formed the reaction mixture products.

Under some conditions, it may be desirable to conduct the reaction mixture directly into zones wherein the valuable major products may be made to react either with themselves or other substances. Frequently, however, it may be desirable to separate the valuable reaction products from the undecomposed isobutylene and the less valuable reaction products. In accordance with the present invention this is effected by conducting the reaction mixture of gases and vapors leaving the cracking zone into a zone where the valuable major reaction products may be separated by cooling followed, if necessary, by distillation. By following this separation procedure, it is possible to obtain the amounts of allene and methyl acetylene mentioned in the specific example given.

As another example of the presentinvention l-pentene (CHaCI-IzCHzCH CHz) was passed through a tube similar to that described in connection with the first example and after experimenting to determine the optimum conditions of cracking, it was found that by passing the 1- pentene through the tube while at 740 C. and at a pressure of of an atmosphere and at such a rate that the l-pentene was in contact with the tube for a period of about 0.06 second, there was obtained a yield of butadiene approximating 36% of the pentene decomposed. This desired reaction product may be separated from th reaction mixture by cooling and distillation such as referred to in connection with the preceding example.

While I have given examples of my process starting with butylene and pentene, other olefins such as propylene and higher members of the olefin series may be used. In this connection it may be stated that any normally gaseous aliphatic olefin having more than three carbon atoms, or any light normally liquid aliphatic olefin, or said olefins in admixture with saturated hydrocarbons, may be used.

The temperature employed may be within the usual range of cracking temperatures suitable for cracking olefin hydrocarbons. A range of 700 to 900 C. is effective. The pressure may vary between /20 and /3 of an atmosphere and the contact time may vary from 0.5 to 5.0 seconds,

From the foregoing, it will be appreciated that the present invention provides an effective method of cracking olefins to produce diolefins and acetylenes and a method of separating the desired reaction products from those less desired.

I claim:

1. A method of cracking olefin hydrocarbons comprising passing the olefin hydrocarbons through a cracking zone at a cracking temperature under a sub-atmospheric pressure and at a rate, correlated to maintain a condition just short of that at which a visible amount of tar, liquid, or carbon is present in the reaction products discharged from said zone.

2. A method of cracking olefin hydrocarbons comprising passing the olefin hydrocarbons through a cracking zone under conditions of cracking temperature, pressure and rate of flow approaching, but short of those at which a visible amount of tar, liquid, or carbon would be present in the products discharged from said zone.

'3. A method of determining the conditions of pressure and rate of flow in the cracking of olefin hydrocarbons comprising passing the olefin hydrocarbons through a cracking zone at a cracking temperature, and at a sub-atmospheric pressure, observing the products discharged from the reaction zone, varying the pressure and rate of flow until smoke formation in the products discharged from the reaction one is visible, and then varying the pressure and rate of flow until a condition just short of smoke formation is reached.

4. A method of determining the conditions of pressure and rate of flow in the cracking of olefin hydrocarbons comprising passing the olefin hydrocarbons through a cracking zone under a subatmospheric pressure, observing the products discharged from the cracking zone, and correlating the temperature and pressure in the cracking zone and the rate of flow through the zone to maintain a condition just short of that at which a visible amount of tar, liquid or carbon is present in the products discharged from the cracking zone.

FRANCIS 0. RICE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,880,308 Wulff Oct. 4, 1932; 2,210,117 Dreyfus Aug. 6, 1940 2,236,534 Hasche Apr. 1, 1941 1,732,381 Schmidt et al. Oct. 22, 1929 2,131,089 Beeck et al Sept. 27, 1938 2,190,359 Rice Feb. 13, 1940 2,209,215 Wiezevich et al. July 23, 1940 1,282,906 Mersereau Oct. 29, 1918 2,322,122 Frolich June 15, 1943 OTHER REFERENCES Ostromysslenski, Chem. Zentralblatt 1, 780-781 (1916). Copy in 260-680.

Eglofi, Reactions of Pure Hydrocarbons, pages 331-840; 350-354.

Ellis, Chemistry of Petroleum Derivatives, vol. II, 57-64, 67-73 (1937). Copy in Division 31.

Egloff et al., Oil and Gas Jour., 229-231, Nov. 12, 1942. Copy in Division 31. 

